Background: Although breast cancer and breast cancer chemotherapy (chemo) have been linked to accelerated functional decline, it is not well understood whether this decline is driven by cancer itself or the combination of cancer and chemo. Here, we compared the change in functional status over time in older breast cancer survivors treated with and without chemo and age-matched women without cancer. Methods: Women age ≥65 with non-metastatic breast cancer (n = 538; 441 treated with chemo and 97 without chemo) and n = 100 non-cancer controls were prospectively evaluated at two timepoints: ≤14 days pre-chemo (baseline) and ≤30 days post-chemo (or matched times for non-chemo and non-cancer controls). At each timepoint, functional status was measured using instrumental Activities of Daily Living (iADL) scores. The primary endpoint was the proportion of patients with a decline in functional status (Yes/No, yes defined as ≥2-point decrease [minimal meaningful difference] in iADL scores between timepoints). Baseline demographic, functional, and clinical characteristics were compared between survivors treated with and without chemo and non-cancer controls using t and chi-squared tests. Among the 441 women treated with chemo, univariate and multivariable logistic regression analyses were performed to determine baseline risk factors associated with chemo-induced functional decline. Results: 10% of older survivors treated with chemo experienced a clinically meaningful decline in function as compared to 3% in the non-chemo and 4% in non-cancer control groups (p = 0.017). Across the 3 groups, there were no differences in median age, race/ethnicity, education, number of comorbidities, or baseline functional status (iADL, ADL, Timed Up and Go [TUG]). Among the 441 older survivors treated with chemo, greater age, higher BMI, more comorbidity, lower ADL score, and longer TUG were significantly associated with functional decline univariately. After multivariable analyses, age ≥78 (26% declined, odds ratio [OR] = 3.67, 95% CI 1.60-8.43) and BMI > 30 (16% declined, OR = 2.11, 95% CI 1.02-4.38) remained significantly associated with functional decline. Patients who were both ≥78 years old and obese (BMI > 30) had the highest odds of developing functional decline post-chemo (41% declined, OR = 8.43, 95% CI 2.48-28.63). Conclusions: In this study, older breast cancer survivors treated with chemo had a 3-fold increased incidence of clinically meaningful decline in functional status as compared to age-matched survivors not treated with chemo and those without cancer. Although these findings need to be replicated in larger studies, our results raise the possibility that accelerated functional decline may be driven by cellular damage from cytotoxic chemo. Further research is warranted to understand the impact of cancer and its treatment on older adults' functional status and underlying aging processes. Clinical trial information: NCT01472094.